Twin Turbos vs. Single Turbos

Tuesday, February 16, 2016

Why would I choose compound turbos over a single turbo?

There are certain facts and laws of nature that work against every diesel driver. Among these are 1. Ambient (atmosphere) air pressure is approximately 14.7 psi at sea level. 2. Turbochargers only function at their best in a limited band of RPM's.

Because of this, most diesel drivers have to choose either:

A small turbo (like a stock turbo) which functions very well with most engines at low RPMs, but limits high end power, or

A large turbo which functions well at wide open throttle (WOT), but is horrible to drive around town because it won’t spool up (takes too long to produce a boost) and has surging issues), or

A medium size turbo which spools up relatively well and at WOT still has some exhaust restriction but allows a lot more power and cooler EGTs than the small turbo.

To help alleviate these problems, DPS offers Turbochargers that offer quick spool up and a good increase in airflow and 10%-13% increase in horsepower over a stock turbo. However, single chargers, even with enhancements, all have limitations.

Why is air pressure a factor for my turbo?

Here is a quick science lesson: If you see boost pressure on your truck at 35 psi, this is actually the gauge pressure (psig). The zero on the gauge is actually 14.7 psi (at sea level), so the actual pressure is what you read on your gauge (35) + 14.7 which equals 49.7 psi.

What does this mean for my turbo?

What this means is that the pressure trying to get into the turbo is only 14.7 psi, and despite how fast you spin the turbo, there is only 14.7 psi pushing air in. So, if you spin the turbo too fast it becomes inefficient at bringing new air in, and it becomes increasingly harder to get exhaust out.

Is there an easy way to fix the air pressure problem?

There are ways to lessen the problem. With a single turbo, people increase the sizes of compressor and turbine wheels as well as the turbine housings. This can increase the amount of airflow, but ultimately hurts low end drivability and spool up, and all because the engine is up against those pesky laws of physics. A certain size of hole (the turbo air inlet) will only flow a limited amount of air at a given atmospheric pressure. Atmospheric pressure becomes a huge limiting factor.

Is there a way to get my turbo to function well at low RPMS, at WOT and without surging or spooling issues?

Compounds are two turbos placed sequentially, one flowing into the other. They must be sized perfectly so they complement each other instead of work against each other. These turbos consist of a smaller charger and a larger charger. The small turbo gets exhaust from the engine first and the large turbo gets fresh air first. Fresh air enters the larger turbo which spools slower. It is then pressurized and fed into the small, quick spooling turbo. The small turbo multiplies the already pressurized air, and then feeds that air into the engine.

For those who enjoy numbers, here is a quick illustration:

Turbos multiply atmospheric pressure. Therefore if the small turbo as a single can take air at 14.7 psi, and produce 40 psi boost, it is multiplying the air by 3.72 times (14.7 psi x 3.72 = 54.7 psia, minus the 14.7 atmospheric gives 40 psig (gauge pressure)). The large turbo can do a similar job. Therefore let's say that the large turbo multiplies by 2.2 times, it takes 14.7 psi (atmospheric pressure) and makes 17.6 psig (actual pressure 32.3 minus 14.7 atmospheric, not taking into account adiabatic efficiencies), now the small turbo will see 32.3 psia at it's air inlet (instead of the 14.7 psia, that it would normally see as a single turbo), but the secondary/small turbo thinks it's only seeing atmospheric pressure. Then when the secondary turbo multiplies the 32.3 psia by 2.4 times you get about 65 psi. A lot of cool air to help in combustion and carry heat out of the cylinders.

If my engine runs a twin turbo, how will that change things?

The beauty of the whole staged, two turbo concept is this: You can have all of the benefits from a small quick spooling turbocharger, with more-than-all of the benefits of a very large turbocharger.

With a twin turbo, the engine can receive over double the amount of air than a single turbo, would be able to move because that’s all atmospheric pressure would allow. Because with twins we can literally cram over double the volume of air into the same inlet hole size in the small turbo (we have just turbocharged the small turbo).

How does this affect the efficiency of my engine?:

Efficiency is the amount of energy which is converted into heat during the compression process. The higher the efficiency the less heat is made from compressing the air. Most compressor maps for turbos end between a 3.5:1 and 4:1 ratio because the efficiency of the compressor drops beyond that point so dramatically. Most compressors have their highest efficiency between 1.5:1 and 2.5:1 ratio. Our DPS 62mm will run at 78% efficiency below 2.3:1 ratio, which is where it typically will run on twin turbos, running appx. 55-60 psi boost. Whereas at 40 psi boost when run as a single turbo the ratio is 3.7:1 and the efficiency drops to 70%, or 8% less than at 2.3:1 ratio (which is still very efficient at that level compared to most other turbos but is significantly lower than the twins at a much higher boost level, in contrast-the stock turbo at this same ratio is 5% less than that). By 45 psi most single turbos are running off the compressor map at efficiency below 68%. In other words our Twin Turbo Kit at 55 psi boost is running much more efficiently than any single turbo running at only 40 psi boost.

More efficient engines mean more usable, cool air entering the engine. To show the increase in compressing efficiency, our Twin Turbo Kit at 58 psi of boost produces compressed air temps (before the intercooler) of appx. 375 degrees F, while at 40 psi the stock turbo produces air temps of appx. 495 degrees F.

Our Twin kits actually put extra air molecules into the cylinder which are not consumed during combustion, but do carry heat out of the cylinder, keeping things much cooler. This is similar to the bypass ratio used on jet engines to keep the exhaust stream cool.

How does the exhaust to boost pressure affect my engine?

The exhaust to boost pressure ratio is the ratio of the exhaust pressure (pre-turbo), to the boost pressure. You want the exhaust and boost pressures to be as close to equal, or 1:1 as possible. Single turbos rarely can do this. For example, the stock turbo you typically see is about 70 PSI exhaust pressure at 40 psi boost pressure.

How do DPS Compound Kits improve my exhaust to boost pressure?

Compared to our compound kits, at 60 psi of boost pressure, we typically see 58-60 PSI of exhaust pressure. This results in direct horsepower gains, because the power required to push the exhaust out, is regained by the high boost pressure, which pushes the piston down on the intake stroke, with our Compounds. This adds horsepower.

Because of the high psi produced by our Compound Kits, the wastegate completely opens about 2/3 the way up the boost scale. By allowing the small turbo to wastegate like this, it also lowers the exhaust (drive) pressure. Again helping with power and EGT’s.

What are your final thoughts on Twin Turbos?

With a lot more cool air in the cylinder, lower exhaust drive pressure, and quick spool-up, there is plenty of air for top-end performance, yet great drive and tow ability. A properly engineered Compound/Twin Turbo Kit, will translate into much more horsepower, much lower EGT's, better fuel mileage, a much broader RPM range (quick spool up, with huge high end WOT potential), and, overall, a much more drivable, high performance truck. All of this while the turbos don’t have to work as hard, making them more reliable and able to last much longer. So, next time you see a company trying to sell a single turbo, claiming it will compete with our Twins, remind yourselves that the laws of physics just don’t allow it. Our customers all say that once they have installed our Twin Turbo Kits, they could never go back to a single turbo.